In a central site installation, a large number of remote users access a central computer system or data base. They may use data communication devices (DCDs) operating over a variety of network services such as PSTN (public switched telephone network), leased line analog service, leased line digital services such as DDS and fractional T1, and switched digital services such as ISDN (integrated services digital network), Datapath, and switched 56 K services.
A large number of connections to the telephone network are required, as well as a large number of communication ports on the host central computer. FIG. 1 shows a prior art communication system. A LAN (local area network), WAN (wide area network) or mainframe computer 10 is connected to a multi-port I/O (input/output) concentrator 12. A rack 14 of analog modems 16 is connected to the concentrator 12. Each analog modem 16 has a separate connection to the concentrator 12. The rack 14 is connected to a 1.544 megabits per second (Mbps) T1 channel bank 18 (usually provided with analog voice cards). The T1 channel bank 18 is coupled to public switched telephone network 20 by T1 line 22. Remote modems 24 coupled the remote user's systems 26 to the public switched telephone network 20 by way of analog lines 28.
Such a system has a number of problems. Reconfiguration is difficult because all of the components must be cable together individually. Changing a T1 time slot assignment or replacing a failed modem would require physically moving devices (such as cards) and cabling. Redundancy is available, but only at the expense of providing additional host ports and telephone network connections.
A system that reduces the number of telephone network connections and host system ports while providing simplified reconfiguration and redundancy is thus highly desirable.